The invention relates to a support mean for apparatus that is mounted in operating position within a housing relative to the center line of which the concentricity of the apparatus must not shift when thermal cycling of the housing and apparatus causes relative movement in a radial direction between those members and causes differential thermal expansion between those members in a longitudinal direction. More particularly, a preferred application of the invention relates to a particle beam position monitoring apparatus that is mounted concentrically within a calibration ring and is provided with a support means that permits relative radial movement and relative thermal growth or contraction in a longitudinal direction between the monitoring apparatus, the calibration ring and the housing, while maintaining a required degree of close concentricity between the components themselves and the housing.
In the design of particle beam position monitoring apparatus it has long been recognized that the accuracy and reliability of beam position signals produced by the monitors is subject to random positional variations of components caused by thermal cycling of the apparatus. Such thermal cycling inevitably occurs because the position monitoring components in some instances must be vacuum fired at about 950.degree. C. and subsequently they are periodically baked to approximately 300.degree. C. in order to achieve ultra high vacuum conditions. A common type of beam position monitor is configured as a split-plate or split-cylinder mounted adjacent to a particle beam in a manner such that the split-plates generate electrical signals due to capacitive coupling of each plate to the charged particle beam. Because the plates of such position monitoring apparatus are split diagonally relative to the longitudinal center line of the surrounded beam, the resultant signal on each plate is proportional to the position of the beam with respect to the center line of the plates. Accordingly, it is important to prevent a change in concentricity of the split plates with respect to one another as well as with respect to the adjacent cylindrical components, while enabling the plates to move radially and to thermally expand in a longitudinal direction relative to the beam line when the apparatus is subjected to thermal cycling.
Another design problem concerning such split-plate beam position monitors and their associated support means is that the beam position signals detected by the plate must be brought out through the housing of the particle beam line without compromising either the ultra high vacuum requirements or the concentricity-maintaining function of the support for the position monitor detector plates and the associated calibration apparatus mounted within the beam line housing.